This invention relates to a detection system for sensing an object in motion relative to a container, especially tubular in design, whereby at least one magnetic unit is associated with the container and/or object, generating as well as measuring magnetic fields, and at least one evaluation device is connected to the magnetic units and serves to receive sensing signals from the magnetic units.
A detection system of this type is described in U.S. Pat. No. 3,103,976. That particular detection system is used in locating pipes, and especially pipe ends to be joined, in underwater drilling and similar operations. A guide tube, serving as a container extending between a topside derrick and a frame section anchored on the sea bottom, is equipped on its outside with a coil as the magnetic unit generating a magnetic field and with each two search coils respectively mounted above and below the first coil and serving as the magnetic-field measuring magnets. Electric cables connect these various coils with a topside evaluation unit within the derrick. The magnetic-held-generating coil produces a magnetic field inside the guide tube essentially along the longitudinal axis of the tube. That magnetic field also permeates the two magnetic-field-measuring coils. If and when within the guide tube a drill rod, tool, pipe or the like is shifted, the magnetic field in these measuring coils will change as a function of the position of the moving object, leading to a corresponding induction in these coils. It is thus possible to determine when the object concerned has reached one of these magnetic-field-measuring coils or for instance the blowout valve located on the sea bottom.
That earlier detection system, however, is essentially limited to sensing the position only of the forward end of the moving object, with the positional detection accuracy being determined by its distance from the coils which are mounted along the longitudinal axis of the guide tube, by the coil width in the longitudinal direction, and similar factors.